1. Field of the Invention
This invention relates to a system for inspecting low pressure, low flow distribution pipelines for defects in structural integrity used in the natural gas pipeline distribution field.
2. Description of Prior Art
There are hundreds of thousands of miles of low pressure, low flow steel distribution pipelines currently in operation. All pipelines are prone to time dependent defects, such as corrosion, which can reduce safety, undermine security of service and threaten the environment if failure occurs. Prudent operators recognized the need to inspect these pipelines to ensure that affected locations are repaired or replaced before failure occurs.
In the mid-1960's, systems were developed to inspect high pressure transmission pipelines. These inspection devices are commonly referred to as "pigs" ("intelligent pigs," "smart pigs"). While there are several technologies currently used for this inspection application, the first developed, and still the most common, is that of Magnetic Flux Leakage (MFL). However, these have heretofore only been used on a frequent and reliable basis in high-pressure environments. Thus, there has been a need for an inspection system developed for low pressure distribution pipelines that exploits MFL technology.
Traditional pipeline inspection tools are free swimming devices that travel through the pipeline with the flowing product. A seal is formed to the wall of the pipeline by flexible cups attached to the tool. A differential pressure across this sealed cup creates the force preferred to propel the tool. Magnetic flux leakage inspection tools generally have two or more segments coupled together by a flexible joint.
The MFL inspection technique is well established and generally easy to apply in high pressure applications. It comprises inducing a high level magnetic field into the wall of the pipeline under inspection and scanning the inside surface with a magnetic sensor to detect variations in the magnetic field caused by wall thinning defects or other imperfections or features which change the expected magnetic properties of the material.
A typical MFL inspection tool contains a magnetic section to induce a magnetic field into the pipe wall. This section can comprise either permanent magnets or electromagnets. The magnetic poles (North/South) are separated axially along the pipe and connected to the pipe by hard metallic pole pieces or flexible wire brushes. The pipe between the poles forms part of the magnetic circuit and can be saturated with magnetic field. Sensors are mounted mid-way between the poles and radiate circumferentially around the pipe. These sensors scan the inside wall of the pipe as the tool is moved through the pipeline.
Another module on the tool contains electronic systems to process and store information acquired by the sensor. Additional modules contain batteries and control systems preferred for tool operation.
Low pressure, low flow distribution pipelines present a unique challenge to the application of this technology. The pressure and flow are not adequate to propel the tool. The inspection system should operate in an in-service pipeline without disrupting flow. The system should minimize wall contact to reduce contamination of the product with debris or deposits which may reside on the pipe wall. The system should be small and flexible to negotiate minor changes in bore and normal pipe bends. The system should operate at very low power levels in order to be certified for gas operation. The system should provide the method of gaining access to the pipeline to be inspected under live conditions; inserting the inspection device into the pipeline; propelling the inspection module through the pipeline and withdrawing the module at a single location. The system should consider bi-directional operation from a single entry point and be portable (trailer mounted) to allow operation in congested city areas. The inspection should be fast and the inspection data available in real time.
Thus, a problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them are not capable of self-propulsion, as the pressure and flow are inadequate to propel the tool.
Another problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them could disrupt the flow of the product in the pipeline in this application.
Yet a further problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them contact the pipe wall excessively, thereby causing contamination of the product with debris or deposits which may reside on the pipe wall.
Still another problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them are not small and flexible enough to be inserted into the main through the off-take.
An even further problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them do not operate at sufficiently low power levels to be certified for gas operation.
Another problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them do not provide a method of gaining access to the pipeline to be inspected under live conditions.
Yet another problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them are not insertable into the pipeline through a side off-take.
Still a further problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them comprise an inspection module which cannot be propelled through the pipeline and withdrawn from the pipeline at a single location.
Another problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them do not facilitate bi-directional operation from a single entry point in the pipeline.
A further problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them are not portable to allow operation in congested city areas.
An additional problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them do not provide for a rapid inspection.
Yet another problem associated with systems for inspecting in-service gas distribution mains that precede the present invention is that many of them do not furnish inspection data available in real time.
For the foregoing reasons, there has been defined a long felt and unsolved need for a system for inspecting in-service gas distribution mains that can be propelled through a low pressure, low flow pipeline and yet can provide real time, reliable data as to the condition of that pipeline.
In contrast to the foregoing, the present invention constitutes a system for inspecting in-service gas distribution mains that seeks to overcome the problems discussed above, while at the same time providing a simple, easily constructed apparatus that is readily adapted to a variety of applications.